Active Nodes
Active routers: EDGE-1, EDGE-4, CE-CORP-1, CE-CORP-2, CORE-1
Key Concepts
Stub Area — Blocking External LSAs
In a stub area, the ABR blocks Type-5 (external) LSAs and instead injects a Type-3 default route (0.0.0.0/0) into the area. Stub routers use this default for all external destinations. A totally stubby area additionally blocks Type-3 inter-area LSAs — only intra-area routes and the ABR default are present. Configure with area 100 stub no-summary on the ABR (totally stubby). All routers in the area must be configured as stub: area 100 stub.
NSSA — Redistributing External Routes in Stub
NSSA (Not So Stubby Area) allows a stub area to contain an ASBR that redistributes external routes, which would normally be prohibited in a stub. The ASBR generates Type-7 LSAs (only within the NSSA). The ABR translates Type-7 to Type-5 LSAs when advertising to the backbone. This is useful when a CE site has both a VPN connection (via PE) and a direct internet connection — the CE can redistribute its default route as a Type-7 without breaking the stub topology.
Type-3 LSA Filtering
ABRs can filter specific Type-3 (inter-area summary) LSAs using area X filter-list prefix FILTER-NAME in or out. The "in" direction filters LSAs entering the area from the backbone; "out" filters LSAs the ABR generates into the backbone from the area. This is the only place in OSPF where you can influence which inter-area prefixes are visible — OSPF does not support filtering within an area (intra-area routes are always flooded completely).
Area Configuration
router ospf 10 vrf CORP router-id 10.255.2.1 area 100 stub no-summary network 10.1.100.0 0.0.0.1 area 100 area 100 filter-list prefix FILTER-INTERNAL in ip prefix-list FILTER-INTERNAL deny 10.255.0.0/16 le 32 ip prefix-list FILTER-INTERNAL permit 0.0.0.0/0 le 32
router ospf 10 router-id 10.255.6.1 area 100 nssa network 10.1.100.0 0.0.0.1 area 100 network 192.168.100.0 0.0.0.255 area 100 redistribute static subnets